Rudimentary description of the receptors for putative spinal cord inhibitory neurotransmitters on chick sensory neurons grown in culture has recently been given. The aim of this project is to further characterize the physiology and pharmacology of these presynaptic receptors and begin to define, through receptor binding studies and autoradiographic techniques, aspects of receptor density and distribution. Of particular interest is the possible correlation between changes in the physiological response during neuronal development in culture and alterations in receptor binding, density, and/or distribution. There is evidence that a closer examination of presynaptic receptors will provide a key to understanding the precise mechanism of presynaptic inhibition in the vertebrate spinal cord.